The present invention relates to digital communications and, in particular, to methods and systems for processing variable length messages.
In a wide variety of digital communication systems, the data (e.g., voice, facsimile, e-mail messages, computer files, etc.) that is transmitted via the communications link and/or network is configured into data frames prior to transmission. Data frames that are transmitted over the system may include part or all of a particular message, and the data frames transmitted by a particular terminal during a particular communication may be of the same length. Digital communications protocols which incorporate such a frame structure may operate under a variety of different multiple access techniques, specifically including frequency division multiple access (xe2x80x9cFDMAxe2x80x9d), time division multiple access (xe2x80x9cTDMAxe2x80x9d) and code division multiple access (xe2x80x9cCDMAxe2x80x9d) techniques, or combinations thereof.
In various communications systems which transmit messages via the use of data frames, control messages are also sent over the regular communications channel. These control messages are configured into xe2x80x9ccontrol framesxe2x80x9d that are inserted into the stream of data frames that is transmitted over the channel. This may be done, for example, by replacing all or part of a data frame with one or more control frames that contain control message(s). When such replacement is performed, the regular communications frame which is replaced by the control frame(s) may be (i) delayed and transmitted immediately after the control frames or (ii) queued and retransmitted at a later time. However, with real time communications, such as voice communications, the delay and queuing options may not be desirable, in which case the regular communications frame(s) that was replaced by control frame(s) may be discarded.
An example of a wireless communications system that interrupts the flow of regular communications traffic to send control messages are systems that operate under the IS-2000 air interface. The IS-2000 interface is described in the publication entitled xe2x80x9cPhysical Layer Standard for edma2000 Spread Spectrum Systemsxe2x80x9d, the disclosure of which is incorporated by reference herein. Pursuant to the IS-2000 air interface, voice and data traffic are carried over the fundamental channel via a steady stream of 20 millisecond data frames. Periodically, this stream of 20 millisecond data frames is interrupted so that 5 millisecond control message bursts (xe2x80x9ccontrol framesxe2x80x9d) may be transmitted that are used for maintaining the communications link. Under the IS-2000 standard, a control frame is sent at one of the 5 millisecond boundaries that divides a 20 millisecond data frame into four equal sub-intervals. Both the 20 millisecond data frames and the 5 millisecond control frames have cyclic redundancy check (xe2x80x9cCRCxe2x80x9d) bits for error detection and are convolutionally encoded before transmission. Thus, for example, a 20 millisecond frame might contain 176 data bits and 16 CRC bits (192 total bits), which are then encoded using a rate xc2xd convolutional encoder to result in a frame having 384 bits (2xc3x97192) that is transmitted over the IS-2000 fundamental channel.
In embodiments of the present invention, methods and systems are provided by which a terminal may decode a received data frame of a first length which includes an embedded control frame of a second length. Pursuant to one embodiment of these methods and systems, the terminal receives a data frame, and then examines at least part of this data frame to determine if it includes an embedded control frame. If an embedded control frame is detected, the terminal replaces that portion of the received data frame that corresponds to the embedded control frame with a dummy data sequence to provide a modified data frame, which is then decoded. If an embedded control frame is not detected, the terminal may decode the received data frame without modification.
Pursuant to other embodiments of the present invention, a receiver determines the length of a data frame within a burst and a transmitter integrates a control message into a data stream. Additionally, in yet other embodiments of the present invention communications terminals and systems for receiving/decoding data frames which include embedded control frames are provided.